The N means NORMAL gasoline in Germany with 92 octane.
The S means SUPER with 95 octane.
For the so called NORMAL Benzin (gasoline) the ignition timing is retard by approx. 10 degrees later to prevent knocking or ping which can destroy your engine. If you use the N position you will decrease also a little bit of power and increase a little bit the fuel consumption. Therefor
the best in my guess will be the S position and the use of 95 octane fuel.

Michael Vogt
86, 420SL type 107